High dielectric constant cavity resonator



June 10, 1958 J. H. FOSTER 2,838,736

HIGH DIELECTRIC CONSTANT CAVITY RESONATOR I Filed Nov. 13, 1953 2 Sheets-Sheet 1 FIG 2 M /3/ FIG. A1

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BY W W I ATTORNEY June 10,-.1958 J. H. FOSTER 2,333,736

HIGH DIELECTRIC CONSTANT CAVITY RESONATOR Filed Nov. is, 1955 1 r 2 Sheets-Sheet 2 INVENTOR ATTORNEY HIGH DIELECTRIC CONSTANT CAVITY RESONATOR James H. Foster, Erie, Pa., assignor to Erie Resistor Corporation, Erie, Pa., a corporation of Pennsylvania Application November 13, 1953, Serial No. 391,955

2 Claims. (Cl. 333--83) This application is a continuation-in-part of my application Serial No. 343,660, now abandoned, filed March 20, 1953.

In cavity resonators, the circuit constants depend upon the physical dimensions of the cavity which normally will be of the order of the wave-length at which the cavity is resonant. This invention is intended to produce a cavity resonator in which the cavity, instead of containing a low permittivity material such as air, comprises a solid high dielectric constant dielectric which may for example be one of the titanates having a dielectric constant of from 1600 to 5000 or more. Since the physical dimensions vary inversely as the square root of the dielectric constant, the high dielectric constant cavity resonator can be quite small. For example, in the 100200 megacycle range, the high dielectric constant cavity resonator can be a disc having a diameter of the order of one inch and a thickness of 10-20 mils or more, these dimensions of course varying with the resonant frequency, because like all cavities the resonant frequency is related to the dimensions and the dielectric constant and permeability. An air-filled metal cavity for the 100-200 megacycle range would be two or more feet in diameter. The production of small sized high dielectric constant cavity resonators opens up a field of use in electronic circuits such as television receivers where the cavities can be used as frequency selective and frequency determining elements. The high dielectric constant cavity resonators can be excited electromagnetically and the exciting loop provides a path for direct current.

In the accompanying drawing, Fig. 1 is a top plan view of one type of electromagnetically excited cavity resonator; Fig. 2 is a section on line 22 of Fig. 1; Fig. 1a is a top plan view ofanother type of electromagnetically excited cavity resonator dittering in the construction of the exciting loop; Fig. 2a is a section of the Fig. 1a resonator; Fig. 3 is a top plan View of a modification; Fig. 4 is an edge view of the Fig. 3 resonator; Fig. 5 is a top plan view of another modification; Fig. 6 is a section on line 66 of Fig. 5; Fig.7 is a diagram of the equivalent lumped circuit for the cavity resonator; Fig. 8 is a diagrammatic view of a cavity resonator having a voltage sensitive dielectric controlling the frequency response of the resonator; Fig. 9 is a side elevation of another electromagnetically excited cavity resonator; Fig. 10 is a top plan view of the Fig. 9 resonator; and Fig. 11 is a side view of another way of exicting the Fig. 10 resonator.

In Figs. 1 and 2 is shown a resonator having a high dielectric constant disc 1, for example, one of the titanates such as barium or strontium titanate having a dielectric constant varying from 1600 to 5000 or more, which may for example have a diameter of the order of one inch and a thickness of the order of 10-20 mils or greater. On opposite faces of the disc are metal electrodes 2 and 3 which may for example be silvered electrodes fired on by the technique used in decorating ceramics. As shown in Fig. l, the electrodes 2 and 3 have re-entrant portions 4 in register with each other. In the ceramic disc there ited States Patent 7' will are a pair of spaced holes 5 and 6, the hole 5 being arranged within the re-entrant portion 4 and the hole 6 being arranged outside the re-entrant portion 4. Both the holes 5 and 6 have rnetallized coatings 7 and 8. At one end of the holes 5 and 6, the coatings 7 and 8 are respectively connected to leads 9 and 10. At the other end of the holes 5 and 6, the coatings 7 and 8 are connected by a coating 11. The leads 9 and 10 are for connection of the resonator to an electric circuit. The coatings 7, i1, and 2, form an exciting loop having its magnetic field directed circumferentially around the periphery of the disc 1 between the electrodes 2 and 3. The loop is located in a region at which theelectromagnetic field intensity is high and the electromagnetic energy coupled into the dielectric disc 1 causes the same sort of field distribution within the dielectric as is present in the conventional hollow air-filled metallic cavity resonator. The electrodes 2 and 3 serve as boundaries preventing radiation of energy from the surface of the dielectric disc. The electrodes 2 and 3 can be connected over the edge of the dielectric disc as indicated by dotted lines in Fig. 2 at the reference numeral 12 and this will still further confine electrical energy within the dielectric disc and improve the efiiciency. If the electrodes 2 and 3 are omitted, the device will function as a cavity resonator, there being a sutficient boundary discontinuity between the high dielectric constant disc (1600-5000) as compared to the dielectric constant of air (1 approximately) to confine the major portion of the electric energy to within the dielectric disc and thereby obtain the same type of cavity resonator operation but at a lower efficiency and diiferent frequency depending on the thickness of the disc due to the absence of the metallic conducting boundary electrodes 2, 3, and 12.

While the cavity resonator has electrical characteristics which depend (in part) upon the dimensions of the dielectric disc, the approximate equivalent circuit is that shown in Fig. 7 and consists of a condenser 13a and an inductance 14a. equivalent circuit shown in Fig. 7 cannot be identified with particular elements in the cavity resonator. The Q of the cavity resonator depends upon the design and the cavity dielectric characteristics and can easily be in the range of from 60 to or higher. By having the metal coatings 7, 8, and 11 directly applied to the ceramic disc 1 by the silver paint technique used in decorating ceramics, dimensional stability is obtained so the exciting loop of the resonator is in fixed spaced relation and in intimate contact with the high dielectric constant ceramic so that the electromagnetic coupling is substantially constant. By providing two angularly spaced exciting loops, in the Fig. 1 resonator, for example spaced apart, one of the loops can be the input and the other the output in which case the approximate equivalent circuit will resemble a transformer with its primary and secondary tuned.

In Figs. 1a and 2a is shown a high dielectric constant resonator which differs primarily in the construction of the electromagnetic exciting loop. In this resonator, the ceramic disc 1 has a metallized coating 13 extending over almost the entire surface area of opposite faces of the disc and also over the edges of the disc. Thereis a re-entrant portion 14 in the coating 13 within which is located a hole 15 having a silvered coating 16 fired in the bore thereof. On oposite faces of the disc within the re-entrant portion 14 are radial stripes 17 and 18 respectively connected to opposite ends of the coating is and forming therewith an electromagnetic exciting loop whose magnetic field is directed circumferentially around the periphery of the disc and within the cavity provided by the coating 13. The stripes 17 and 13 are'rcspectively connected to leads i9 and 20 by which the resonator can The condenser and inductance in the be connected into an electronic circuit. The loop 16, 17, and 18 provides a direct current path between the leads 19 and 26 which adapts the resonator for use as a high frequency filter.

In Figs. 3 and 4 is shown a high dielectric constant ceramic resonator in which the disc 1 has an arcuate e'xciting electrode 21 silvered on one face thereof and connected to leads 22 and 23. The magnetic field of the electrode 21 is co-axial with the disc at the surface rather than circumferentially around the periphery of the disc. It is important with this arrangement that electrode coatings be kept off the opposite faces of the ceramic disc 1, as these electrode coating would prevent electromagnetic coupling with this method of excitation. Because the efiiciency of coupling is less and boundary leakage or radiation is greater than in the previously described construction, the efliciency of the resonator is lower.

In Figs. and 6 is shown another form of electromagnetically excited high dielectric constant resonator in which the disc 1 has substantially semi-circular electrode coatings 24 and 25 extending over opposite faces of the disc and over the edges, except for an insulating band 26, 27 extending diametrically across the center of the disc. At the center of this insulating band is an elecromagnetic exciting loop comprising stripes 28 and 29 joined at one end over an edge of the disc and having the other ends terminating within the periphery of the disc. By connecting leads to the spaced ends of the stripes 26 and 29, electromagnetic excitation can be provided. The magnetic field of the stripes 28 and 29 is directed into the cavity defined by the metal coatings 24 and 25.

In Fig. 8 is shown a re-entrant type resonator in which the frequency response is controlled by a voltage sensitive dielectric section 39 which, for example, may be one of the barium titanate strontium titanate mixes at the center of an annulus 40 of high dielectric constant ceramic of the type which is not sensitive to voltage or temperature. The voltage sensitive section 39 has its Curie point below the normal operating temperature. At opposite faces of the voltage sensitive section 39 are electrodes 41 and 42, which are connected to leads 43 and 44 across which may be impressed a control voltage. At suitable points Within the annular dielectric 40 may be arranged electromagnetic exciting loops diagrammatically indicated at 45 or 46 arranged so that the electromagnetic field is directed circumferentially around the annular ceramic body 41 A metal coating 47 may be applied over the outer surface of the annular body 40. The Fig. 8 resonator functions in the same manner as the other resonators, but has the possibility of altering the frequency response due to the change in dielectric constant of the section 39 resulting from the control voltage applied across the leads 43, 44.

In Figs. 9 and is shown a more efficient version of the resonator illustrated in Figs. 3 and 4. In this form, a cylinder 49 of high dielectric constant ceramic having a length substantially the same as the diameter has at its mid-section a metallic band 50 connected to leads 51 and 52. Although the magnetic field of the band 50 is directed axially, the relatively great axial depth of the ceramic body 49 provides a relatively high efiiciency as indicated by a Q of from 6680 or higher.

In Fig. 11 is shown another-arrangement for providing electromagnetic excitation for the Fig. 9 cavity resonator which will have higher elficiency than the band 50. In this arrangement, an arcuate slot 53 is ground in the ceramic cylinder 49 and the bottom of the slot is provided with a fired on silver coating 54 which acts as an electromagnetic exciting loop. The leads to the loop coating 54 are connected to the ends, as in the previously described constructions. The slot 53 extends radially inward toward the center of the cylinder and axially along the length of the cylinder.

In all the resonators, there is an electric field distribution within the dielectric which is of the same general configuration as in the hollow air-filled metallic cavity resonators. A substantial amount of this electric energy is retained Within the dielectric body due to the reflection at the outer surface of the dielectric body resulting from the discontinuity introduced by the relatively high dielectric constant of the body as compared to air. Because the physical size of the resonator is inversely proportional to the square root of the dielectric constant, these resonators can be of such small physical dimensions as to be useful as filters in high and ultra-high frequency electronic circuits. While the frequency response of the resonators depend upon the physical dimensions, these dimensions can be held practically to close enough limits permitting the use of the resonators as filter elements.

In making connections to external circuits for exciting electromagnetic waves in cavity resonators and for absorbing energy from them, a loop may be used to provide coupling mainly with the magnetic field within the cavity.

To obtain maximum coupling with the magnetic field, the plane of the loop must be orthogonal to the magnetic intensity vector within the cavity. To obtain a maximum r linkage of magnetic flux with the loop, the correct position may be obtained by a study of the field configuration within the resonator. The field distribution will be influenced somewhat by the presence of the loop and this effect can be lessened by making the loop small in comparison with the wavelength of the waves in the cavity. Thus, the larger loop type of coupling specified in Fig. 5 would cause larger changes in field configuration than the small loop thereby causing a non-uniform current distribution within the cavity and increasing the losses.

The selection of the ceramic dielectric will depend upon the use of the resonator. If frequency stability is required, the ceramic may be a barium titanate having a K of the order of 1600 and having a negligible voltage coetficient of capacity, a fiat temperature characteristic (:5% over the range 50 C. to C.) and low aging (1% per decade). If the wider variation in frequency response can be tolerated, one of the higher K mixes such as those including calcium and strontium titanate can be used. When the frequency response is to be controlled by auxiliary electrodes connected to a source of control voltage, a mix having a pronounced voltage coefficient of capacity may be selected such as one of the barium strontium titanates. Although the control of the frequency response by varying the dielectric constant is specifically illustrated in Fig. 8, such control could be used with any of the other forms having electrodes on opposite faces of the dielectric provided there was no electrical connection between the electrodes.

What is claimed as new is:

1. A cavity resonator comprising a high dielectric constant ceramic dielectric plate, electrodes metallized on opposite faces of the plate, spaced holes through the dielectric outside the electrodes, metallized coatings in the holes, terminals connected to one end of the metallized coatings in the holes, and a loop connection to the other ends of the metallized coatings in the holes, said loop connection being unconnected to the electrodes but electrically coupled thereto through the dielectric by directing its magnetic field between the plates.

2. A cavity resonator comprising a high dielectric constant ceramic dielectric plate, electrodes metallized on opposite faces of the plate, at least one hole through the dielectric outside the electrodes, a loop having ends spaced from each other and comprising series connected metallized. coatings on at least one face of the dielectric and on the inner surface of said hole, terminals connected to the spaced ends of the loop, said loop connection being unconnected to the electrodes but electrically coupled 6 thereto through the dielectric by directing its magnetic FOREIGN PATENTS field between the Plates- 976,767 France Nov. 1, 1950 References Cited in the file Of this patent OTHER REFERENCES UNITED STATES PATENTS 5 Journal of Applied Physics, v01. 24, No. 2, February,

2,583,854 Kehbel Jan. 29, 1-952 1953, pages 187-191. (Copy 333-78.) 2,611,094 Rex Sept. 16, 1952 2,639,324 Haa'vey May 19, 1953 2,704,830 Rosencrans Mar. 22, 1 955 

1. A CAVITY RESONATOR COMPRISING A HIGH DIELECTRIC CONSTANT CERAMIC DIELECTRIC PLATE, ELECTRODES METALLIZED ON OPPOSITE FACES OF THE PLATE, SPACED HOLED THROUGH THE DIELECTRIC OUTSIDE THE ELECTRODES, METALLIZED COATINGS IN THE HOLES, TERMINALS CONNECTED TO ONE END OF THE METALLIZED COATINGS IN THE HOLES, AND A LOOP CONNECTION TO THE OTHER 